Lecture 12 OPTICL DETECTOS (eference: Optical Electronics in Modern Communications,. Yariv, Oxford, 1977, Ch. 11.) Photomultiplier Tube (PMT) Highly sensitive detector for light from near infrared ultraviolet Can detect as little as 10 19 Watt! vacuum envelope D1 D3 D5 D7 C D2 D4 D6 D8 HV ~2 KV Voltage divider chain to bias dynodes chosen to have ~100V drop per dynode Photocathode C: absorbs photon ejects electron work function is the minimum energy needed to eject an electron the photon energy must exceed the work function to get photoelectrons Dynodes D1-D8:Secondary electron emission. Electron from cathode accelerated by ~100 ev. Impact into dynode surface causes ejection of multiple electrons, 5. For N dynodes, the total gain is then N. Photocathode quantum efficiency: 0 QE 1 Typical photocathode response QE QE probability a photon will eject one electron 30% 20% 10% 200 400 600 800 1000 (nm) Sensitivity: 1 Jeffrey Bokor, 2000, all rights reserved
For 10 dynodes, 5 G 5 10 10 7. Take 2eV photons (620 nm), 1 picow 10 12 W 10 12 J/s With QE 30% 1 ev 1.6 10 19 J, so, 10 12 W 3 106 photons/s, node current is Phototube dark current:1) random thermal excitation of electrons from photocathode 2) cosmic rays, ambient radioactivity Thermal excitation rate is proportional to e kt, where represents the cathode work function so lower work function I sensitivity, but larger dark current For room temperature, typical cathode dark current, I cd, is 10 4 electrons/sec. node dark current is then I ad I cd G Dark current sets a lower limit to phototube sensitivity to low light levels. To distinguish a light signal above the background dark current, the photoelectric cathode current must exceed the dark current. If is 10 4 e/sec, then the sensitivity to light can be 3 10 4 photons/sec (assuming QE I cd 30%). 3 10 4 red photons/s 10 14 W! 3 10 6 0.3 10 7 10 13 electrons/s 1.6 10 6 Dark current can be reduced by cooling. Using thermoelectric cooling T 40C is easily obtained. ssume a work function of 1.5 ev I cd 260K ------------------------ e 1.5 0.0225 I cd 300K --------------------------- e 1.5 0.026 e 1.5 44.4 38.5 e 8.8 1.4 10 4 Dark current is reduced by this amount! down to ~1 e/sec. Minimum detectable power become < 10 18 W! Photon counting: PMT is so sensitive, we are really counting photons. Often, PMT circuits are specifically optimized to do this. 2 Jeffrey Bokor, 2000, all rights reserved
Photon counting system: PMT anode amplifier/discriminator pulse counter 50 C PMT output pulse discriminator threshold V p p 3-5 ns p r t transit time dispersion 2V discriminator output is a digital pulse t How big is the PMT output pulse from one photon? For G 10 7, we get 10 7 electrons 10 12 C. For p 10 8 sec, I apk 10 4. For 50, V p 5 mv. Discriminator eliminates electrical noise in < 1mV range. V p has a variation due to statistical nature of gain process. Discriminator also eliminates this. Shot noise: Photon arrival is always statistical. Generally it follows Poisson statistics. Then if the photon arrival rate is N ph/sec, and we count for 1 sec, we get N on average. The standard deviation will be found to be N. This means we have noise. N counts N Signal/Noise ratio N N clearly depends on counting time N sec 1 2 3 4 5 6 7 Shot noise is universal for light detection. Even if photons are not explicitly counted, the shot noise is a fundamental limit. It is most significant at low light levels, though, due to N dependence. Johnson noise: andom thermal noise in any resistor, 3 Jeffrey Bokor, 2000, all rights reserved
I MS 4kT -------- B V MS 4kTB ~ I MS B: bandwidth (Hz) ~ V MS Equivalent model of noise as either current or voltage source. Channel Electron Multiplier (Channeltron) Single monolithic device functions as a PMT: HV 1 2 electron cascade 1. Photon hits funnel portion 2. Electrons are accelerated into the bent tube by bias field 3. Secondary electron emission gives gain at each electron collision with wall 4. Must be operated in vacuum 5. Typical gain 10 4 6. More compact and rugged than PMT Microchannel plate MCP array of channeltrons Glass tube bent around curve. One end open as a funnel shape. Coating acts as photocathode and secondary electron emitter. lso, coating has high, but not infinite electrical resistance. 10-20 m separation each hole ~5-10 m diameter each channel is a miniature channeltron gain ~10 3 node gain ~10 6 HV ~1-2KV HV Dual Plate MCP Detector 4 Jeffrey Bokor, 2000, all rights reserved
MCP Image Intensifier phosphor-coated plate e light eye or video camera HV Single or Dual Plate MCP Electrons accelerated out of back of MCP into phosphor Phosphor QE ~50% photons/electrons Image intensification QE MCP G MCP QE phosphor 10 5 Night vision goggles 5 Jeffrey Bokor, 2000, all rights reserved